The preflight test layers of NASA's James Webb Space Telescope sunshield are meeting expected performance targets during tests by engineers at Northrop Grumman Corporation (NOC). The company is under contract to NASA's Goddard Space Flight Center in Greenbelt, Md., for the design and development of Webb's sunshield, telescope and spacecraft.
One of the most important test milestones was successfully met for template layer five of the tennis-court-sized sunshield that keeps the telescope cold so it can image faint infrared light. Using a laser tracking instrument and a laser radar unit, engineers carefully measured the 3-D shape of the tensioned test layer in two different orientations: face up and rotated 180 degrees so it was face down. They then compared the measurements to an analytical model that predicted how the ultra-thin material will behave in an environment close to zero gravity.
"The as-built and measured membrane was within .36 inches Root-Mean-Squared of the 3-D shape the model predicted, over an area as large as a tennis court, which is remarkable," explained Jim Flynn, Webb sunshield manager, Northrop Grumman Aerospace Systems. "Our teammate, ManTech International, has done an outstanding job in sunshield fabrication and test. This result validates our ability to manufacture the sunshield layers to meet extremely demanding performance standards."
The sunshield membrane layers, each as thin as a human hair, are made of Kapton, a tough, high-performance plastic coated with a reflective metal. On-orbit the observatory will be pointed so that the sun, Earth and moon are always on one side, with the sunshield acting as an umbrella to shade the telescope (the mirrors and instruments) from the heat of the sun and warm spacecraft electronics. The sunshield passively cools the telescope to a temperature of -375 degrees F, which is needed to prevent the observatory's own heat from "blinding" its infrared sensing instruments.
Template layer five is the coldest layer, has the most curved shape and is closest to Webb's 21 ft. diameter primary mirror. Each sunshield layer has a slightly different 3-D shape, much like the petals of a flower. Each layer will be individually shape-tested to verify its performance on orbit. Shape testing is also under way for two of four template sunshield covers. These covers are coated with silicon to reduce launch and ascent temperatures and protect the folded sunshield layers when they are stowed in the Ariane 5 rocket. Engineers are using the template or test layers to validate processes and performance before fabricating the flight sunshield layers.
Qualification testing was also completed on the gearmotors or actuators that drive the mechanisms that unfurl the sunshield layers while Webb travels to its orbit nearly 1 million miles from Earth. These gearmotors are subjected to tough tests to simulate the effects of extreme temperature changes, vibrations, operating loads and performance over the life of the unit. There are six motors: two drive the sunshield mid-boom telescoping tubes that unfurl the sunshield horizontally out into space; two drive the spooler that opens the two shells that hold the folded layers; and two are used to create the tension that holds the layers in place. Successful completion of qualification testing for the gearmotors demonstrates the engineering design and allows flight hardware manufacturing to proceed.
The James Webb Space Telescope is the world's next-generation space observatory and successor to the Hubble Space Telescope. Webb will be the most powerful space telescope ever built, observing the most distant objects in the universe, providing images of the first galaxies ever formed and studying planets around distant stars. The Webb telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency.
For more information about the Webb telescope, visit: www.jwst.nasa.gov.
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